This proposal concerns the composition and control of circadian rhythms in mammals. Our goal is to understand at the molecular and cellular level the function and tissue specificity of components comprising the circadian oscillator. In mammals, circadian rhythms in physiology and behavior are regulated by an endogenous clock. The circadian clock mechanism consists of two autoregulatory feedback loops, the PER/CRY core loop and the BMAL1 interlocking loop. In the core loop, the activators BMAL1 and CLOCK activate the transcription of Per and Cry genes; PER and CRY proteins in turn repress their own transcription through direct interaction with BMAL1/CLOCK. Recent experimental evidence suggested that a second interlocking BMAL1 loop serves as a stabilizing mechanism that contributes to the robustness of the circadian clock. For the rhythmic expression of BmaM, REV-ERBa acts to repress BmaM transcription. Our gene discovery program recently generated data to show that RORa acts as a positive driver to activate Small expression in the SCN. The central clock in the SCN integrates environmental cues and coordinates oscillators in peripheral tissues. We propose to define the role of the related protein RORc in peripheral circadian oscillators. We will further extend the study on the RORs and REV-ERBs to address their respective contributions to the BmaM loop. This will serve as a new entry point to explore the heterogeneity and distinct functions of the SCN and peripheral clocks. Finally we will focus on the liver oscillator to address how the liver clockworks participate in local physiological processes. Better understanding of the circadian clockworks and peripheral oscillators, will expand our understanding of the mechanisms that underlie diurnal variation in the incidence of clinical events and therapeutic response. This would be expected to refine diagnostic and therapeutic strategies and to reveal novel clock-related disorders. Presently, these include sleep disorders, shift-worker fatigue, jet lag, cancer, myocardial infarction and stroke, neurological disorders, and seasonal depression.